The invention relates to an image projection device comprising a display system having at least a display panel for generating an image to be projected, a projection lens system for projecting the image formed by the display system on a projection screen, a focus error detection system provided with an auxiliary radiation source which supplies a focus measuring beam, and a radiation-sensitive detection system for converting focus measuring beam radiation reflected by the projection screen into a focus error signal.
The term image projection device is considered to have a wide meaning and comprises a device for displaying, for example a video image, a graphic image, numerical information or a combination thereof. The images may be both monochrome and color images. In the latter case the display system may comprise three chrominance channels for, for example the primary colors red, green and blue, each accommodating a display panel. A display panel may be constituted by the display screen of a cathode ray tube but is preferably a liquid crystalline panel. In the latter case the display system comprises an illumination unit for illuminating the panel or the panels.
In the current image projection devices which comprise one projection lens system in the form of a zoom lens for projecting a magnified image on a projection screen which is present at, for example several meters from the projection device, each change of the projection distance or each change of the image size on the screen necessitates refocusing of the image by readjusting the zoom lens manually or possibly via a remote control unit. Moreover, optical elements of the image projection device may be displaced with respect to each other due to, inter alia temperature variations, so that the projected image may be defocused. The known image projection devices thus require an additional quantity of time, attention and expertise of the user. The convenience of use of an image projection device would be enhanced considerably if it were provided with an autofocus system, i.e. a system with which the distance between a display panel and the projection screen is measured and the focal length of the projection lens system is automatically adjusted with reference to said measurement.
An image projection device in which an auxiliary radiation source emits an invisible, infrared measuring beam to the screen and in which the radiation beam reflected by the screen is received by a position-sensitive detector is described in the published Japanese Patent Application (Kokai) 3-149538. The distance between the screen and a reference plane, the plane of the radiation source and/or the detector can be determined by means of this system which is referred to as triangulation system. This information is used to displace the entire projection lens system along the optical axis so that the distance between this system and the screen can be adapted to the focal length of the projection lens system. This system has the drawback that the measuring beam is optically not coupled to the projection lens system and the display panel so that it is not ensured that the focal length of the projection lens system is always adapted to the distance between the screen and the display panel.